The goal of this study is to determine what constraints architectural design places on muscle function in vivo, and how those constraints might affect the pattern of motor recruitment by the nervous system. We will study three muscles in the thigh of the rat during treadmill locomotion. Electromyography, sonomicrometry, high-speed video, and force plate recordings will be used to examine the kinematics and kinetics of the individual muscles, as well as of the limb as a whole. The three muscles chosen are representative of three common architectural designs. The semitendinosus, a series-fibered muscle, the vastus lateralis, a parallel-fibered muscle, and the rectus femoris, a bi-articular synergist of the uni-articular vastus lateralis. The results of this study will clarify the rules governing motor recruitment across muscles of varied anatomy and function.